Polystyrene foams have been used in wide applications because of their excellent performances as a heat insulating material and a packaging and cushioning material. However, they have a low recovery of compression strain and can withstand heat at a temperature of 70.degree. to 80.degree. C. at the highest. These defects may be eliminated by using polypropylene foams It is difficult, however, to produce foamed particles suitable as a starting material for polypropylene foams since blowing agents used for a polypropylene resin have a high speed of dissipation. Any foamed particles that may be obtained have a low expansion ratio with a bulk density of from 0.1 to 0.5 g/cm.sup.3 at the highest.
In an attempt to solve this problem, there has been proposed a process for producing high-expansion polypropylene foamed particles having a bulk density of from 0.05 to 0.07 g/cm.sup.3, which comprises dispersing polypropylene resin particles containing 10 to 70% by weight of inorganic fillers in water as a dispersing medium in a closed vessel, maintaining the dispersion at a high pressure above the saturated vapor pressure of the dispersion and at a temperature above the softening point but below the melting point of the polypropylene to thereby penetrate the dispersing medium into the polypropylene resin particles, and then jetting the dispersion from the inside of the closed vessel under high pressure into the atmosphere, as disclosed in U.S. Pat. No. 3,770,663 (corresponding to Japanese Patent Publication No. 2183/74).
The above-described process employs water that is used as a dispersing medium and also serves as a blowing agent and is, therefore, advantageous in that foamed particles can be produced at lower cost as compared with the processes using organic blowing agents, such as dichlorofluoromethane, butane, hexane, etc. Nevertheless, the presence of a large quantity of inorganic fillers is unfavorable as inhibiting melt-adhesion among particles during molding of the foamed particles.
On the other hand, a process for producing polyolefin resin foamed particles which comprises dispersing polyolefin resin particles in water in a closed vessel, feeding a volatile organic blowing agent into the closed vessel, heating the dispersion to a temperature above the softening point of the polyolefin resin particles while maintaining the pressure within the closed vessel at the vapor pressure of the blowing agent or a higher pressure for a given period of time, opening a discharge port provided in the closed vessel below the liquid level, and releasing the polyolefin resin particles containing the blowing agent together with water into an atmosphere having a lower pressure than the pressure within the closed vessel has been proposed as described in Japanese Patent Application (OPI) Nos. 12035/82, 25336/82, 90027/82, 195131/82, 1732/83, 23834/83, 25334/83, 33435/85, 55231/83 (EP 75897), 76229/83 76231/83, 76232/83, 76233/83, 76234/83 and 87027/83.
This process can provide foamed particles having a high rate of expansion of from 15 to 50 by using non-crosslinking polypropylene. However, use of a volatile blowing agent leads to an increase in cost for the production of foamed particles.
The present inventors have conducted research in an attempt to obtain polypropylene resin foamed particles having a rate of expansion of from 5 to 20 and uniform size distribution without using a hydrophilic inorganic filler in the aforesaid process disclosed in U.S. Pat. No. 3,770,663 which does not involve use of a volatile blowing agent. As a result, it has been found that expansion of highly crystalline homopolypropylene is difficult and that a propylene-ethylene random copolymer only produces foamed particles having a rate of expansion of 2 to 3 at the most. Further, the thus formed particles have non-uniform size and shape. Non-uniformity of the shape of foamed particles causes non-uniform packing of the particles in molding, resulting in the failure of obtaining molded articles having constant mechanical properties.